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HYBRID MICROWAVE FIBER OPTIC LINKS
John A MacDonald and Allen KatzJohn A. MacDonald and Allen Katz
Linear Photonics, LLC3 Nami Lane, Suite 7C, Hamilton, NJ 08619
609-584-5747
macdonald@linphotonics.com
LINEAR PHOTONICS, LLC
June, 20081
Bringing Performance to Light!
Hybrid Photonics• LPL has developed Hybrid Microwave Photonic Link
Components– Directly-modulated Microwave-over-fiber extended to Ku Band– Very small Size and Weight– Lower Cost & Complexity than external modulationLower Cost & Complexity than external modulation– High Dynamic Range: equivalent to external modulation
Microwave Photoreceiver
June, 20082Microwave Laser Transmitter
Directly Modulated Links• Directly-Modulated (DM) Fiber Optic Links
– Transmitter: RF pre-amplification, biasing, and matching to low-impedance laser diode G1
F
RF IN
G1 F
RF IN
matching to low impedance laser diode• Output is Intensity Modulated• Modulation Efficiency = ratio of peak output modulation
envelope to peak microwave input current – Receiver: PIN photodiode matching from high-
F1
Laser Diode
F1
Laser Diode
Receiver: PIN photodiode, matching from highimpedance diode, RF post-amplification
• Responsivity = ratio of generated current to incident light intensity
• PIN diode performs direct envelope detection
OPTICAL OUTPout
Link Length
OPTICAL OUTPout
Link LengthPIN diode performs direct envelope detection
OPTICAL INPinOPTICAL INPin
OLGdBG refL ⋅−= 2)(Link Gain, GL OLGdBG refL ⋅−= 2)(Link Gain, GL
)(174)/()( dBGHzdBmNdBF Lout −+=Link Noise Figure F )(174)/()( dBGHzdBmNdBF Lout −+=Link Noise Figure F
G2 F2
Photodiode
G2 F2
Photodiode)()()( Lout
⎥⎥⎦
⎤
⎢⎢⎣
⎡++=
−−10
21010 101010log10
OLcOLcc
out
rinshotth
N
Link Noise Figure, Fand
Output Noise Power Density, Nout
)()()( Lout
⎥⎥⎦
⎤
⎢⎢⎣
⎡++=
−−10
21010 101010log10
OLcOLcc
out
rinshotth
N
Link Noise Figure, Fand
Output Noise Power Density, Nout
⎤⎡ + eqeq IIPOIP ⎤⎡ + eqeq IIPOIP
June, 20083
RF OUTRF OUT⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
+
= +
1010
10
3
1010
10log10)(Leqeq
eqeq
GIIPOIPdBmIIPLink Input Intercept, IIP3⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
+
= +
1010
10
3
1010
10log10)(Leqeq
eqeq
GIIPOIPdBmIIPLink Input Intercept, IIP3
Packaged Laser Limitations• Low Cost Commercial Laser Transmitters utilize
packaged laser diodes– Widespread use in CATV, RF-over-Fiber
• Thermal and Frequency Limitations– Frequency Response Limited by package parasiticsFrequency Response Limited by package parasitics– Thermal Operation limited by changing gain slope
– TEC-cooled “butterfly” packages solve thermal problem– At expense of even lower bandwidth (more package effects)At expense of even lower bandwidth (more package effects)
– Uncooled “TO-Can” packages can operate to ~ 4 GHz– Slope change limits thermal range for stable link gain to ~ 0 to 50 C
Cooled “butterfly” laserFrequency Response < 3 GHzHigh Current (TEC)Low Reliability (TEC)
Uncooled “TO” laserFrequency Response < 4 GHzLimited Thermal Range
June, 20084
y ( )
Laser Diode Thermal OperationCyoptics 293BT - S/N 448
3.500
4.000
Laser L-I curve over Temperature
-20C
Cyoptics 293BT S/N 449 - S21 Gain - LO2
1
0
1
8 GHz Link Gain over Temperature
1 000
1.500
2.000
2.500
3.000
al O
utpu
t Pow
er (m
W)
70C
-5
-4
-3
-2
-1
Gai
n (d
B)
-0.500
0.000
0.500
1.000
0 10 20 30 40 50 60 70 80 90
Laser Current (mA)
Opt
ica
-9
-8
-7
-6
8.000 8.050 8.100 8.150 8.200 8.250 8.300 8.350 8.400 8.450 8.500
Frequency (GHz)
-20 de g C 0 deg C +25 deg C +50 deg C +75 deg CSlope efficiency and Threshold change with temperature
-20 deg C, 50 mA 0 deg C, 50 mA +25 deg C, 50 mA +50 deg C, 75 mA +75 deg C, 100 mAResults in large variation in link gain over temperature
• Link gain = f(Slope Efficiency) Link Gain = f(Optical Power)g ( p y) ( p )• Bias adjust can maintain optical power, but not slope efficiency• Use of TEC maintains laser temperature
• Requires ~ 2 W DC power• Not suitable for high reliability
June, 20085
• Not suitable for high-reliability• TEC MTBF may be less than Laser Diode MTBF
• Hi-Rel, Low Power solution is RF Variable Attenuation
Hybrid Laser TransmitterOPTICAL SECTION
– Uncooled Laser Diode• High Reliability, Low Noise 1310 nm
RF in
• Low DC Power (no TEC)– Optical Alignment to Single Mode fiber
• Focusing Optics and Optical Isolator
MICROWAVE INPUT SECTION MICROWAVECONTROL
+5
+12
MICROWAVE INPUT SECTION– Temperature Compensation Network– Optimal Microwave Matching
• Package parasitics removed
MICROWAVEINPUT
CONTROLCIRCUITPWR MON
• Operation to inherent laser diode frequency range (12 GHz+)– Broadband or narrowband tuning (application specific)– Provisions for Preamp / Predistortion OPTICAL SECTION
Optical outANALOG CONTROL SECTION
– Analog hybrid on ceramic• Power supply conditioning• Constant Current bias loop
Optical out
June, 20086
• Thermal Control• Backfacet telemetry
Transmitter Layout
RF Launch (GPO)Control Electronics
DC Interface
Optical Platform RF Matching& Attenuator
Fiber Pigtail(9/125 SM)
June, 20087
Link PerformanceTypical Performance for Microwave Links
Examples: C-Band and BroadbandLinks available tuned any bandwidth to 12 GHzLinks available tuned any bandwidth to 12 GHz
C-Band BroadbandBandwidth 3.2 to 4.0 GHz 4 to 12 GHzBandwidth 3.2 to 4.0 GHz 4 to 12 GHzOptical Power 6 dBmo 6 dBmoRF Link Gain -12 dB -15 dBGain Variation with Frequency 0.5 dBp-p 2 dBp-pGain Variation with Temperature 2 dBp-p 2dBp-pRF Input/Output Return Loss 10 dB 10 dBInput IP3 32 dBm 28 dBmNoise Figure 29 dB 32 dBSFDR 118 dB (1 Hz) 113 dB (1 Hz)DC PDC Power
@ -10 C@ 25 C@ 70 C
Operational Temperature Range
375 mW450 mW600 mW
-20 to +70 C
June, 20088
Operational Temperature RangeTransmitter Weight (less pigtail)Receiver Weight (less pigtail)
19 grams5 grams
20 to 70 C
Link Performance
C-Band IF Link
June, 20089
Design Flexibility• Link optimization
– Microwave matching design for any bandwidthMicrowave matching design for any bandwidth requirement
– Integrated preamplification for lower noise • Receive side antenna remoting
• Integrated Predistortion Linearization• Integrated Predistortion Linearization– Improves Intermodulation Distortion
• Bandwidth Extension to 20 GHz– Units available 4Q08
June, 200810
Q
Linearity Improvement• Linearization: Methods of improving the linearity of a
nonlinear network– Predistortion Linearization is one technique
• Employs a nonlinear element in the microwave signal path• Operates at instantaneous microwave rate
N t li it d b d l i f db k f df d h– Not limited by delay as in feedback or feedforward approach– Not limited by overly complicated component-count– Limited primarily by microwave matching, preamplifiers, etc.
• Linearizer Technology Inc (Linear Photonics’ sisterLinearizer Technology, Inc. (Linear Photonics sister company) has been manufacturing linearizers and linearized networks for > 15 yrs
T h l i dil li d t fib ti t k– Technology is readily applied to fiber optic networks
June, 200811
Linearizer Technology, Inc.
Predistortion Linearization
-2
-1
0
1
2
3
Output Power
-5
-4
-3
2
-5 -4 -3 -2 -1 0 1
4
Input Power
-1
0
1
2
3 Phase
-2
-5 -4 -3 -2 -1 0 1Input Power
• Nonlinear Device exhibits Gain and Phase Compression
June, 200812
Predistortion Linearization
-2
-1
0
1
2
3
-1
0
1
2
3
4
5
Output Power Output Power
-5
-4
-3
2
-5 -4 -3 -2 -1 0 1
4
-5
-4
-3
-2
-5 -4 -3 -2 -1 0 1
4
Input Power Input Power
-1
0
1
2
3
-1
0
1
2
3 Phase Phase
-2
-5 -4 -3 -2 -1 0 1
-2
-5 -4 -3 -2 -1 0 1Input Power Input Power
• Nonlinear Device exhibits Gain and Phase Compression
• Precede it with another nonlinear device that exhibits gain and phase expansion, in conjugate with the device to be linearized (the linearizer)
June, 200813
p , j g ( )
Predistortion Linearization
-2
-1
0
1
2
3
-1
0
1
2
3
4
5
-2
-1
0
1
2
Output Power Output Power Output Power
-5
-4
-3
2
-5 -4 -3 -2 -1 0 1
4
-5
-4
-3
-2
-5 -4 -3 -2 -1 0 1
4
-5
-4
-3
-5 -4 -3 -2 -1 0 1
3
Input Power Input Power Input Power
-1
0
1
2
3
-1
0
1
2
3
-4
-3
-2
-1
0
1
2Phase Phase Phase
-2
-5 -4 -3 -2 -1 0 1
-2
-5 -4 -3 -2 -1 0 1-5
-5 -4 -3 -2 -1 0 1Input Power Input Power Input Power
• Nonlinear Device exhibits Gain and Phase CompressionNonlinear Device exhibits Gain and Phase Compression
• Precede it with another nonlinear device that exhibits gain and phase expansion, in conjugate with the device to be linearized (the linearizer)
Th d i d t i id l li it
June, 200814
• The desired outcome is an ideal limiter– The linearity of an ideal limiter cannot be improved
Predistortion Linearization
• Result is reduction in IMD:
SFDR i i t d 1 3 (dB) ith IMD• SFDR is impacted 1:3 (dB) with IMD
June, 200815
Results: Gain and Phase Transfer
5.7 dB1.5 dB
Non linearized @ 8 GHz Linearized @ 8 GHzNon linearized @ 8 GHzP1 dB is 5.7 dB from saturationPhase compression rapidly above sat
Linearized @ 8 GHzP1 dB is 1.5 dB from saturationPhase nonlinearity held to < 1° past sat
June, 200816
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